Study finds waning pertussis protection in kids post-vaccination

Mar 11, 2013 (CIDRAP News) – Health officials have suspected a drop-off in protection from the childhood acellular pertussis vaccine as one factor in surging disease levels, and new data today on Minnesota and Oregon kids provide the strongest evidence yet that immunity wanes before they get a booster dose at age 11 or 12.

The new information comes from two states that have good pertussis surveillance and immunization information systems. Researchers from the US Centers for Disease Control and Prevention (CDC), along with partners at the Minnesota and Oregon health departments, published their findings in an early online edition of Pediatrics.

Though pertussis (whooping cough) levels are cyclical, the United States and other countries have experienced increasing numbers of cases in recent years. The CDC has said that pertussis cases are at their highest point since 1955.

Health officials have suspected that waning immunity from the acellular version of the vaccine (DTaP) could be playing a role, especially after they found an unusual illness spike in 13- and 14-year-olds in Washington state's ongoing epidemic.

Researchers have been exploring factors that could be contributing to the increase in cases. In February one group reported that strains of Bordetella pertussis that have adapted to the acellular vaccine have been identified for the first time in the United States.

Teasing out a possible vaccine protection gap has been difficult, because in about 1997 the United States switched from a whole-cell vaccine that had a fairly high rate of minor side effects to an acellular vaccine that caused fewer reactions, possibly at the expense of stronger protection. Kids receive multiple doses of the vaccine, and some youngsters got the whole-cell vaccine for some of their doses.

In today's study, researchers based their findings on kids who received all five doses of acellular vaccine, given at 2, 4, 6, and 15 to 18 months, as well as at 4 to 6 years. Investigators compared incidence rates and pertussis risk in Minnesota and Oregon kids who were born from Jan 1, 1998, to Dec 31, 2005. They tracked disease levels for each of the 6 years after the children received their fifth and final dose.

Their analyses included only cases that were lab confirmed or met the clinical case definition.

The study included 224,378 Minnesota children who had all five doses and 179,011 Oregon youngsters who were fully vaccinated. In the 6 years after vaccination, there were 458 pertussis cases in the Minnesota group and 89 in the Oregon group.

Pertussis rates rose each year for kids in both states. In Minnesota, the cases rose from 15.6 per 100,000 population the first year to 138.4 per 100,000 population in year 6. For Oregon the pertussis incidence was 6.2 per 100,000 in year 1 and 24.4 per 100,000 in year 6. Researchers also found increases in risk ratios for both states.

The group concluded that the findings strongly suggest waning vaccine immunity. They noted that although Minnesota and Oregon had different pertussis rates during the study period, both had similar increases in risk ratios.

The findings add to the growing body of evidence that DTaP protection begins to wane after vaccination, thereby increasing the number of people susceptible to pertussis. "This growing pool of susceptible persons helps to explain the emergence of an increased burden of disease among 7- to 10-year-olds, a group that previously had a low risk of disease, presumably due to partial or complete vaccination with whole-cell vaccines," they wrote.

Because pertussis is contagious, in years of increased circulation, even a modest drop-off in protection can have a big impact, they noted.

Though many factors may be driving the national resurgence of pertussis, the striking and sudden increase in disease among 7- to 10-year-old that began in 2005 and a strong cohort effect seen in national surveillance data suggest that the major factor is the early waning immunity from acellular vaccines.

More studies are needed to estimate vaccine effective and to explore how long the Tdap booster protects those who were vaccinated. Because better vaccines are on the distant horizon, making the most of the current vaccines offers the best protection against pertussis, they wrote.